1. Field of Invention
The present invention relates to a power supply apparatus that is suitable for supplying a predetermined direct-current (DC) voltage to each electrode of a traveling-wave tube, and a high-frequency circuit system which incorporates the power supply apparatus.
2. Description of the Related Art
Traveling-wave tubes or klystrons or the like are electron tubes for amplifying or oscillating a high-frequency signal based on an interaction between an electron beam emitted from an electron gun and a high-frequency circuit. As shown in FIG. 1, a traveling-wave tube, for example, includes electron gun 6 that emits an electron beam, helix 2 serving as a high-frequency circuit for causing interaction between a high frequency signal (microwave) and an electron beam emitted from the electron gun, first collector electrode 3 and second collector electrode 4 for trapping the electron beam output from helix 2, and anode electrode 5 for drawing electrons from electron gun 6 and guiding the electron beam emitted from electron gun 6 into spiral-shaped helix 2.
Electron gun 6 comprises cathode electrode 7 that emits thermal electrons, heater 8 that applies thermal energy to cathode electrode 7 to cause emission of thermal electrons therefrom, and Wehnelt electrode 9 for focusing electrons emitted from cathode electrode 7 to form an electron beam.
An electron beam that is emitted from electron gun 6 is accelerated by the potential difference between cathode electrode 7 and helix 2 and introduced into helix 2. The electron beam travels through the inside of helix 2 while interacting with a high frequency signal that is input from one end of helix 2. After passing through the inside of helix 2, the electron beam is trapped by first collector electrode 3 and second collector electrode 4. At this time, a high frequency signal that has been amplified by an interaction with the electron beam is output from the other end of helix 2.
Although FIG. 1 shows a configuration example in which traveling-wave tube 1 comprises two collector electrodes (first collector electrode 3 and second collector electrode 4), a configuration in which traveling-wave tube 1 comprises only one collector electrode or comprises three or more collector electrodes is also available.
As shown in FIG. 1, a helix voltage (HX) which is a DC voltage that is negative with respect to a potential (HEL) of helix 2 is supplied to cathode electrode 7, a first collector voltage (COL1) which is a DC voltage that is positive with respect to a potential (HK) of cathode electrode 7 is supplied to first collector electrode 3, and a second collector voltage (COL2) which is a DC voltage that is positive with respect to the potential (HK) of cathode electrode 7 is supplied to second collector electrode 4. Further, an anode voltage (A) that is a DC voltage that is positive with respect to the potential (HK) of cathode electrode 7 is supplied to anode electrode 5, and a heater voltage (H) that is a DC voltage that is negative with respect to the potential (HK) of cathode electrode 7 is supplied to heater 8. Helix 2 is normally connected to the case of traveling-wave tube 1 and is grounded.
The helix voltage (HK), first collector voltage (COL1), and second collector voltage (COL2) are generated using transformer 31, inverter 32 that is connected to a primary winding of transformer 31 and that converts a DC voltage supplied from outside into an alternating-current (AC) voltage, rectifying circuits 33, 34, and 35 that convert an AC voltage output from the secondary winding of transformer 31 into a DC voltage, and rectifier capacitors C11 to C13 that smooth a DC voltage that is output from rectifying circuits 33 to 35.
The anode voltage (A) and Wehnelt voltage are also generated using the inverter, transformer, rectifying circuits and rectifier capacitors in the same manner as described above. The heater voltage (H) is normally generated using the inverter, the transformer, and the rectifying circuits, without using the rectifier capacitors.
The traveling-wave tube shown in FIG. 1 is capable of controlling the amount of electrons emitted from cathode electrode 7 by the anode voltage (A). Therefore, the electric power of a high-frequency signal output from traveling-wave tube 1 can be controlled by the anode voltage (A). For example, even while a high-frequency signal of a constant electric power is being input to traveling-wave tube 1, traveling-wave tube 1 can output a pulsed high-frequency signal by applying a pulsed anode voltage (A) to anode electrode 7. Similar control is also possible using the Wehnelt voltage that is applied to Wehnelt electrode 9 of electron gun 6.
Power supply apparatus 30 shown in FIG. 1 comprises anode switch 36 that supplies or stops the supply of the anode voltage (A) to anode electrode 7, and anode switch control circuit 37 that controls the on/off operations of anode switch 36. Power supply apparatus 30 represents a configuration example in which the pulsed anode voltage (A) can be applied to anode electrode 7.
However, in a high-frequency circuit system as shown in FIG. 1, to prevent damage caused by an excessive current flowing to helix 2 of traveling-wave tube 1 when the power is turned on or turned off, it is necessary to control the order in which the supply of various power supply voltages are turned on and off.
For example, when the power is turned on, first, the heater voltage (H) is supplied to pre-heat heater 8 of traveling-wave tube 1, next, inverter 32 is actuated to supply the helix voltage (HK), the first collector voltage (COL1), and the second collector voltage (COL2), and finally the anode voltage (A) is supplied. In contrast, when turning off the power, first, the supply of the anode voltage (A) is turned off (making the anode voltage (A) equal with the potential (HK) of the cathode electrode), next, the operation of inverter 32 is stopped to turn off the supply of the helix voltage (HK), the first collector voltage (COL1), and the second collector voltage (COL2), and finally the supply of the heater voltage (H) is stopped. The aforementioned anode switch 36 can also be used to supply or to cutoff the supply (stop supply) of the anode voltage (A) when the power is turned on or when the power is turned off. The sequence when the power is turned on or is turned off in this kind of traveling-wave tube 1 is also described, for example, in Japanese Patent Laid-Open No. 8-111183.
In this connection, when supplying a Wehnelt voltage to Wehnelt electrode 9 of electron gun 6, it is sufficient that the Wehnelt voltage be supplied last when the power is turned on, and that the supply of the Wehnelt voltage be stopped first when the power is turned off.
In the above described sequence at the time of stopping the power supply to the traveling-wave tube, when the supply of the anode voltage (A) or Wehnelt voltage is stopped first, since the emission of electrons from cathode electrode 7 stops, the span between each electrode of traveling-wave tube 1 enters a substantially open state. Accordingly, when operation of inverter 32 is stopped to stop supply of the helix voltage (HK), the first collector voltage (COL1), and the second collector voltage (COL2), the helix voltage (HK), the first collector voltage (COL1) and the second collector voltage (COL2) are maintained as they are, since there is no electrical discharge path for electric charges that are accumulated in the rectifier capacitors C11 to C13. In general, since a DC voltage (power supply voltage) supplied to each electrode of traveling-wave tube 1 is between several KV and several tens of KV, when testing or performing maintenance work on traveling-wave tube 1, after stopping the power supply it is necessary to adequately decrease these high voltages using some kind of electrical discharge means.
Since a configuration that has a low current supply capacity is used for a power supply circuit that generates an anode voltage (A) or Wehnelt voltage, even if the anode voltage (A) or Wehnelt voltage remains, the remaining voltage does not constitute a problem. Normally, since a load resistor for stabilizing an output voltage is provided between the output terminals of a power supply circuit that generates the anode voltage (A) or the Wehnelt voltage, when the supply of the anode voltage or Wehnelt voltage stops, an electric charge that is accumulated in a rectifier capacitor is discharged through the load resistor.
In contrast, because a configuration that has a large current supply capacity is used in a power supply circuit that generates a helix voltage or a first collector voltage and second collector voltage, for example, discharge bleeder resistor Rb is provided for each of rectifying circuits 33 to 35 shown in FIG. 1, and electric charges that accumulate in rectifier capacitors C11 to C13 are discharged through discharge bleeder resistors Rb. For discharge bleeder resistors Rb, a comparatively large value (approximately several MΩ) is used for decreasing current that flows at the time of normal operation of power supply apparatus 30.
However, in a configuration that discharges electric charges accumulated in rectifier capacitors C11 to C13 using discharge bleeder resistors Rb, since electric charges are discharged depending on a time constant that is determined based on the values of rectifier capacitors C11 to C13 and values of discharge bleeder resistors Rb, as shown in FIG. 2, there is the problem that time is required until the helix voltage (HK), the first collector voltage (COL1), and the second collector voltage (COL2) decrease sufficiently (approach the potential of the helix (HEL: ground potential)).
As a method for reducing the discharge time of a rectifier capacitor, a method can be considered in which current decreasing resistor Rg is connected to an output terminal of the helix voltage (HK), and the output terminal of the helix voltage (HK) is short circuited with a ground potential through current decreasing resistor Rg using ground rod 38 (see FIG. 1). Alternatively, a method can be considered in which an output terminal of the helix voltage (HK) or a collector voltage is short circuited with a ground potential when operation of the power supply apparatus is stopped by using a high-voltage vacuum relay.
However, since work to short circuit an output terminal of the helix voltage (HK) with ground potential using ground rod 38 involves directly touching a high voltage location, there is a problem that safety decreases when performing such work. On the other hand, although safety when performing work can be ensured in a configuration using a high-voltage vacuum relay, because the cost of a high-voltage vacuum relay is high, the overall cost of the high-frequency circuit system comprising the traveling-wave tube and the power supply apparatus increases.